1. Field of the Invention
The invention relates to an electric power steering system that generates steering assist torque by executing drive control of an electric motor based on a driver's steering operation.
2. Description of Related Art
Conventionally, an electric power steering system detects a steering torque applied to a steering wheel by a driver, and assists a driver's steering operation by driving an electric motor according to a controlled variable set based on the detected steering torque. There is also known an electric power steering system that drives an electric motor using two drive systems that are independent of each other. For example, an electric power steering system described in Japanese Patent Application Publication No. 2011-78230 (JP 2011-78230 A) is configured such that a stator of a three-phase brushless motor has two sets of three-phase coils of two respective systems, and the two sets of three-phase coils are energized independently of each other by two respective inverters. When such a brushless motor is driven, a rotation angle sensor that detects a rotation angle of the motor is used to control the three phases. An electrical angle of the motor is derived based on the rotation angle detected by the rotation angle sensor.
When the rotation angle sensor malfunctions, it is not possible to control the phases of the brushless motor. Therefore, there is also known an electric power steering system that estimates an electrical angle based on an induced voltage (counter-electromotive force) generated at a brushless motor and executes drive control of the brushless motor using the estimated electrical angle when a rotation angle sensor malfunctions. Such control of the brushless motor executed with the use of an estimated electrical angle is called “sensorless control”. This control method is described, for example, in Japanese Patent Application Publication No. 2008-87756 (JP 2008-87756 A).
However, when sensorless control is executed in an electric power steering system in which a brushless motor is driven with the use of two drive systems that are independent of each other, the computation of the induced voltage is doubled and thus the computational load on a microcomputer increases.